The use of superconductors or hyperconductors at very low temperature (e.g. 4.2.degree. K., the temperature of liquid helium) requires current to be conveyed by current leads from a region at ambient temperature (300.degree. K.) to the region at very low temperature.
In general, current leads are made using metal conductors, optionally cooled by the vapor of the cryogenic fluid. The resulting cryogenic load is considerable and it cannot be reduced below a certain limit because of the relationship that exists for metals between electrical conductivity and thermal conductivity (Wiedemann Franz law). Thus, a copper current lead operating between 4.2.degree. K. and 300.degree. K. and optimized for 1000 A, dissipates 1 W in the helium, and uses the resulting vapor for cooling purposes. This vapor represents a cooling and liquefying load of the order of 3 kW of electricity.
As soon as high critical temperature superconductive ceramics appeared (e.g. 93.degree. K. for YBa.sub.2 Cu.sub.3 O.sub.6.9), composite current leads were envisaged comprising a superconductor between the very low temperature (e.g. 4.2.degree. K.) and an intermediate temperature (e.g. 77.degree. K., the temperature of liquid nitrogen) together with a metal conductor between the intermediate temperature and ambient temperature.
A superconductive ceramic has the advantage of producing no heat by the joule effect under DC conditions and of producing very little under certain variable conditions. This advantage is associated with low thermal conductivity.
The article "YBaCuO current lead for liquid helium temperature applications" by F. Grivon et al., 1990 Applied Superconductivity Conference, Snowmass, Colo., 24 to 28 September 1990, describes an example of a current lead where a YBaCuO part is used in the form of a bar or a tube.
For example, it is possible to envisage using a ceramic bar having a section of 20 mm.sup.2, a length of 10 cm, and transporting 1000 A. That would dissipate about 0.2 W in the helium, representing an electrical load of 100 W in a helium refrigerator. This load is in addition to the load represented by cooling the metal current speed between 77.degree. K. and 300.degree. K., which is about 400 W of electricity. Such a solution therefore appears highly advantageous compared with the above-mentioned metal current lead between 4.2.degree. K. and 300.degree. K.
Solid HT.sub.c superconductive ceramic bars have been assembled together as described in the article by J. L. Wu et al. and J. R. Hull et al. in the 1990 Applied Superconductivity Conference, Snowmass, Colo., 24 to 28 September 1990.
In prior publications, a solution has been found for making contact between YBaCuO superconducting ceramic and a copper conductor. Proposals are made in the article by F. Grivon et al. to use a brush to paint a suspension containing silver on the YBaCuO ceramic, which painted suspension is then dried and subsequently subjected to heat treatment at about 930.degree. C. Low contact resistance is thus obtained, of the order of 10.sup.-13 .OMEGA.m.sup.2. A silver-copper bond is then made.
To date, the problem of making a bond between YBaCuO ceramic and a conductor comprising multifilament superconductive strands based on niobium-titanium has not been solved. Under such circumstances, it is quite possible to envisage making a first copper-YBaCuO bond separately from a second copper-multifilament strand bond, with the copper parts then being soldered together (the copper may be replaced by any other suitable metal). Nevertheless, the presence of an intermediate part made of metal having resistance and extending between the YBaCuO ceramic and the superconductive strand will give rise to heat losses from the liquid helium.
An object of the present invention is to provide a connection method enabling losses to be reduced as much as possible, while nevertheless be simple to implement.